M-derived Filter
m-derived filters or m-type filters are a type of electronic filter designed using the image method. They were invented by Otto Zobel in the early 1920s. This filter type was originally intended for use with telephone multiplexing and was an improvement on the existing constant k type filter. The main problem being addressed was the need to achieve a better match of the filter into the terminating impedances. In general, all filters designed by the image method fail to give an exact match, but the m-type filter is a big improvement with suitable choice of the parameter m. The m-type filter section has a further advantage in that there is a rapid transition from the cut-off frequency of the pass band to a pole of attenuation just inside the stop band. Despite these advantages, there is a drawback with m-type filters; at frequencies past the pole of attenuation, the response starts to rise again, and m-types have poor stop band rejection. For this reason, filters designed using m-type sections are often designed as composite filters with a mixture of k-type and m-type sections and different values of m at different points to get the optimum performance from both types.
| Midpoint impedance |
| The parameter m is given this symbol because of its association with midpoint impedance, a concept used by Zobel in his original treatment of the subject. Midpoint impedance arises in the following way. In this article and most modern textbooks, the starting point is the simple half-section, and more complex filters are built up from this. In Zobel's treatment and that of his contemporaries, the starting point is always the infinite ladder network. A "mid-series" section is derived by "cutting through the middle" of the series impedance Z and results in a T section. The image impedance ZiT is referred to as the mid-series image impedance. Similarly, a "mid-shunt" section is derived by cutting through the middle of the shunt admittance Y and results in a Π section with a mid-shunt image impedance. A "series m-derived section" is shorthand for "mid-series derived ladder type section". This makes it clear that the word series is referring to the ends of the T section being (half) a series component and not as is sometimes thought, because the additional component is in series with the shunt element. Similarly, "shunt m-derived section" is shorthand for "mid-shunt derived ladder type section". |
Read more about M-derived Filter: Background, Derivation, Cascading Sections, See Also